2317.5.2 Static and instance methods

Chapter 17 Classes

1• If R contains the introducing virtual declaration of M, then this is the most derived implementation

2of M.

3• Otherwise, if R contains an override of M, then this is the most derived implementation of M with

4respect to R.

5• Otherwise, the most derived implementation of M with respect to R is the same as the most derived

6implementation of M with respect to the direct base class of R.

7[Example: The following example illustrates the differences between virtual and non-virtual methods:

8using System;

9class A

10{

12public virtual void G() { Console.WriteLine("A.G"); }

13}

14class B: A

15{

17public override void G() { Console.WriteLine("B.G"); }

18}

19class Test

20{

28}

29}

30In the example, A introduces a non-virtual method F and a virtual method G. The class B introduces a new

31non-virtual method F, thus hiding the inherited F, and also overrides the inherited method G. The example

32produces the output:

33A.F

34B.F

35B.G

36B.G

37Notice that the statement a.G() invokes B.G, not A.G. This is because the run-time type of the instance

38(which is B), not the compile-time type of the instance (which is A), determines the actual method

39implementation to invoke. end example]

40Because methods are allowed to hide inherited methods, it is possible for a class to contain several virtual

41methods with the same signature. This does not present an ambiguity problem, since all but the most derived

42method are hidden. [Example: In the following code

43using System;

44class A

45{

46public virtual void F() { Console.WriteLine("A.F"); }

47}

48class B: A

49{

50public override void F() { Console.WriteLine("B.F"); }

51}

52class C: B

53{

54new public virtual void F() { Console.WriteLine("C.F"); }

55}

285

C# LANGUAGE SPECIFICATION

1class D: C

2{

3public override void F() { Console.WriteLine("D.F"); }

4}

5class Test

6{

16}

17}

18the C and D classes contain two virtual methods with the same signature: The one introduced by A and the

19one introduced by C. The method introduced by C hides the method inherited from A. Thus, the override

20declaration in D overrides the method introduced by C, and it is not possible for D to override the method

21introduced by A. The example produces the output:

22B.F

23B.F

24D.F

25D.F

26Note that it is possible to invoke the hidden virtual method by accessing an instance of D through a less

27derived type in which the method is not hidden. end example]

2817.5.4 Override methods

29When an instance method declaration includes an override modifier, the method is said to be an override

30method. An override method overrides an inherited virtual method with the same signature. Whereas a

31virtual method declaration introduces a new method, an override method declaration specializes an existing

32inherited virtual method by providing a new implementation of that method.

33The method overridden by an override declaration is known as the overridden base method. For an

34override method M declared in a class C, the overridden base method is determined by examining each base

35class of C, starting with the direct base class of C and continuing with each successive direct base class, until

36an accessible method with the same signature as M is located. For the purposes of locating the overridden

37base method, a method is considered accessible if it is public, if it is protected, if it is protected

38internal, or if it is internal and declared in the same program as C.

39A compile-time error occurs unless all of the following are true for an override declaration:

40• An overridden base method can be located as described above.

41• The overridden base method is a virtual, abstract, or override method. In other words, the overridden

42base method cannot be static or non-virtual.

43• The overridden base method is not a sealed method.

44• The override declaration and the overridden base method have the same return type.

45• The override declaration and the overridden base method have the same declared accessibility. In other

46words, an override declaration cannot change the accessibility of the virtual method.

47An override declaration can access the overridden base method using a base-access (§14.5.8). [Example: In

48the following code

49class A

50{

286

3}

4}

5class B: A

6{

11}

12}

13the base.PrintFields() invocation in B invokes the PrintFields method declared in A. A base-

14access disables the virtual invocation mechanism and simply treats the base method as a non-virtual method.

15Had the invocation in B been written ((A)this).PrintFields(), it would recursively invoke the

16PrintFields method declared in B, not the one declared in A, since PrintFields is virtual and the run-time

17type of ((A)this) is B. end example]

18Only by including an override modifier can a method override another method. In all other cases, a

19method with the same signature as an inherited method simply hides the inherited method. [Example: In the

20following code

21class A

22{

23public virtual void F() {}

24}

25class B: A

26{

28}

29the F method in B does not include an override modifier and therefore does not override the F method

30in A. Rather, the F method in B hides the method in A, and a warning is reported because the declaration does

31not include a new modifier. end example]

32[Example: In the following code

33class A

34{

35public virtual void F() {}

36}

37class B: A

38{

40}

41class C: B

42{

44}

45the F method in B hides the virtual F method inherited from A. Since the new F in B has private access, its

46scope only includes the class body of B and does not extend to C. Therefore, the declaration of F in C is

47permitted to override the F inherited from A. end example]

4817.5.5 Sealed methods

49When an instance method declaration includes a sealed modifier, that method is said to be a sealed

50method. A sealed method overrides an inherited virtual method with the same signature. A sealed method

51shall also be marked with the override modifier. Use of the sealed modifier prevents a derived class

52from further overriding the method.

53[Example: The example

54using System;

287